Method for producing bis-[4-(diphenylsulfonic)phenyl]sulfide bis-MF6 photoinitiator

ABSTRACT

A highly effective photoinitiator for cationic polymerization of monomer formulations is prepared by reacting bis-[4-(diphenylsulfonio)phenyl] sulfide bis-halide wherein halide is chloride or bromide in an aqueous solution with a compound having the formula T MF 6  wherein M is one of phosphorus, antimony or arsenic and T is hydrogen or a metal whose halide salt is water soluble. The two compounds react in the aqueous medium to produce a precipitate which includes at least about 50 percent by weight bis-[4-(diphenylsulfonio)phenyl]  sulfide bis-MF 6 , and this precipitate may be recovered and dried for utilization without further purification if so desired. Purification may be effected by dissolving the dried product in an organic solvent, cooling the solvent to precipitate the impurities, and thereafter recovering relatively pure bis-[d-(diphenylsulfonio)phenyl]  sulfide bis-MF 6  by chilling the solvent solution to form a precipitate, and then separating and drying the precipitate. The preferred compounds are those wherein the substituent M is phosphorus so that the product comprises bis-[4-(diphenylsulfonio)phenyl]  sulfide bis-hexafluorophosphate. The cationic polymerization processes utilize the photoinitiator in an amount equal to about 0.5-6.0 percent by weight of the monomer therein.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a division of my copending application Ser.No. 20,340 filed Mar. 14, 1979, now U.S. Letters Pat. No. 4,201,640.

BACKGROUND OF THE INVENTION

For a number of years, the polymer coating industry has been engaged insubstantial development programs involving the identification orsynthesis of various photoinitiators which would permit high speedcuring of the polymerizable formulations following exposure to sourcesof electromagnetic radiation. This has been particularly true in varioussegments of the coating industry where it has been desired to eliminatethe volatile solvents required in many polymer coating processes becauseof the potential hazards or because of the cost of the equipment tohandle the evolved solvent vapors. However, the industry has longrecognized the necessity to provide polymerizable compositions whichwould exhibit relatively long pot life and/or shelf stability prior toexposure to the source of electromagnetic radiation.

In William R. Watt U.S. Pat. No. 3,794,576 granted Feb. 26, 1974, thereare described and claimed certain highly advantageous epoxy coatingformulations which combine the desired rheological properties withsuitable pot life and rapid curing. The formulations therein utilize asthe photoinitiator diazonium salts which decompose upon irradiation byultraviolet light to produce rapid curing of the coating to a tack-freecondition. As explained in detail therein, the desired rapid curing ofthe epoxy formulations requires that they contain at least about 15percent by weight of an epoxidic ester with two epoxycycloalkyl groups.

Since the disclosure of the Watt Patent, there have appeared a number ofpatents and publications disclosing onium catalysts effective to replacethe diazonium catalysts specifically described in the Watt Patent. Theseonium catalysts are described in detail in Barton U.S. Pat. No.4,090,936 granted May 23, 1978, and Crivello U.S. Pat. Nos. 4,069,055granted Jan. 17, 1978 and 4,058,401 granted Nov. 15, 1977. The mechanismand operation of the triaryl sulfonium salts is described in detail inCrivello et al "Triaryl Sulfonium Salts: A New Class of Photoinitiatorsfor Cationic Polymerization", JOURNAL OF RADIATION CURING, Volume 5,pages 2, 10-11, January 1978, and "UV Curing: Science and Technology",edited by S. P. Pappas (Technology Marketing Corporation, Stamford,Connecticut).

Among the most popular of the sulfonium catalysts for epoxypolymerization and for other cationic polymerization is triphenylsulfonium hexafluorophosphate. Many of the recent studies have centeredupon the use of this photoinitiator. Although triphenyl sulfoniumhexafluorophosphate offers certain advantages over the diazonium typecatalysts from the standpoint of longer shelf life and rapid curing, itis generally a relatively expensive material which has a limited rangeof spectral sensitivity. Accordingly, there has continued to be asignificant need for relatively low cost, high speed photoinitiatorsaffording relatively broad spectral sensitivity to electromagneticradiation and which would also exhibit highly desirable shelf life.

Accordingly, it is an object of the present invention to provide a novelprocess for producing a highly effective photoinitiator for cationicpolymerization of various monomers.

It is also an object to provide such a process which enables utilizationof relatively inexpensive and readily available reactants and whichutilizes simple procedures for reaction and for purification if sodesired.

SUMMARY OF THE INVENTION

It has now been found that the foregoing and related objects may bereadily attained in a method in which a photoinitiator is prepared by aseries of steps including the preparation of a first reactant comprisingbis-[4-(diphenylsulfonio)phenyl] sulfide bis-halide wherein halide ischloride or bromide and admixing the first reactant in an aqueoussolution with a second reactant having the formula T.MF₆ wherein M isone of phosphorus, antimony or arsenic, T is hydrogen or a metal whosehalide salt is water-soluble. The two reactants in the aqueous mediumreact to produce a precipitate which includes at least about 50 percentby weight bis-[4-(diphenylsulfonio) phenyl] sulfide bis-MF₆ based uponsolids content, and this precipitate is recovered and then dried,initially at a temperature below about 50° C. to obtain a frangibleproduct. This product may then be dried further at a temperature of upto about 110° C.

To purify the crude product thus obtained, it is then dissolved in asolvent therefor selected from the group consisting of C₁ -C₃ alcohols,C₃ -C₆ ketones, and C₂ -C₆ esters at an elevated temperature in excessof 50° C. and up to the boiling point of the solvent. The solventsolution is cooled to form an insoluble precipitate, and the supernatantsolution is separated from the precipitate. The solution is then chilledto a temperature below about 10° C. effective to precipitate out arelatively pure product which is then recovered and dried to produce aphotoinitiator comprising about 90-98 percent by weightbis-[4-(diphenylsulfonio) phenyl] sulfide bis-MF₆.

In the preferred embodiment of the present invention, the halide of thefirst reactant is chloride. The phenyl groups of the first reactant andaccordingly of the photoinitiator are unsubstituted; alternatively, theymay be halogen substituted. In the preferred embodiment, M is phosphorusand T is hydrogen or an alkali metal.

Desirably, the solvent is methanol and the temperature of the solutionis lowered to between 0° and -25° C. to precipitate the relatively pureproduct. In dissolving the initial product in the methanol solvent, atemperature of 50° to 65° C. is employed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As previously indicated, the method of the present invention involvesthe reaction of two reactants in an aqueous medium to produce aprecipitate which includes bis-[4-(diphenylsulfonio) phenyl] sulfidebis-MF₆ and this precipitate is subsequently dried and desirablypurified to recover a photo-initiator which is substantially comprisedof bis-[4-(diphenylsulfonio) phenyl] sulfide bis-MF₆.

The first reactant is bis-[4-(diphenylsulfonio)phenyl] sulfidebis-halide wherein halide is chloride or bromide and the phenyl groupsthereof may be unsubstituted or they may be substituted with halogens.Such compounds are available commercially from various sources or theymay be conveniently synthesized by known and conventional processes.Preferably, the reactant is relatively purebis-[4-(diphenylsulfonio)phenyl] sulfide bis-halide wherein halide ischloride or bromide, and the preferred compounds are those where thehalide is chlorine.

The second reactant is a compound having the formula T.MF₆ wherein T ishydrogen or a metal whose halide salt is water-soluble, and M isselected from the group consisting of one of phosphorus, antimony andarsenic. The preferred reactants are those wherein T is hydrogen orsodium or potassium and wherein M is phosphorus. Again, such compoundsare available commercially, and they also may be synthesized readily.

The two reactants are admixed in an aqueous medium in which they areboth soluble, preferably in a ratio of two moles of TMF₆ per mole ofbis-[4-(diphenylsulfonio)phenyl] sulfide bis-halide wherein halide ischloride or bromide since their reaction is substantially quantitative.Although an excess of T MF₆ may be employed if so desired to ensurecomplete reaction of the bis-[4-(diphenylsulfonio)phenyl] sulfidebis-halide wherein halide is chloride or bromide, it must be appreciatedthat an excess of either reactant will be unused and therefore increasethe costs of the process and the problems of purification.

The water into which the reactants are introduced must be of sufficientquantity to permit them to be dissolved therein and to permit thegelatinous precipitate which forms to be separated from the dissolvedsalts and other impurities. Generally, the amount of water may be withina broad range of 0.5-2.0 liters per mole of T MF₆ so long as the desiredseparation may be effected. The temperature of the water and thereby thetemperature at which the reaction is conducted does not appear to beparticularly significant either to the reaction itself or to theformation and nature of the gelatinous precipitate since no appreciabledifferences have been noted at temperatures ranging from 0° C. to 50° C.Stirring for a limited period of about 0.5-2.0 hours is desirable toensure complete reaction. Since the reaction proceeds almostinstantaneously, time is controlling only from the standpoint ofpermitting the gelatinous precipitate to form and to lose some of thewater entrapped therein during its formation. Normally, a period of0.5-2.0 hours following reaction is desirable for this purpose.

As indicated above, the gelatinous precipitate containing the productalso includes a substantial volume of water which must be removedtherefrom and the entrapped water may contain some of the undesiredsalts and impurities. Accordingly, preliminary steps to remove some ofthe water are desirable including vacuum filtration or filtration in afilter press.

The drying operation is quite critical since initial exposure totemperatures in excess of about 50° C. will convert the precipitatedproduct to an amorphous, dark colored syrup which on cooling becomes abrittle, glass-like material which is very difficult to process further.Accordingly, the material should be dried in air or under vacuum slowlyat ambient temperatures until the material becomes friable, i.e., breaksinto a crumbly product when pressure is applied with a spatula or a rod;this will normally require a period of 12-36 hours. Following thisinitial drying, the temperature may be elevated to a temperature notexceeding about 110° C. for further drying during a period of about 4-18hours; generally, a temperature of 100°-105° C. for a period of 6-8hours is satisfactory. Dessicants such as phosphorus pentoxide areadvantageously employed.

Although the product may be used in this state, it may be desirable totake further steps to effect purification to increase the amount ofbis-[4-(diphenylsulfonio) phenyl] sulfide bis-MF₆ in the product. Atthis point, the bis-[4-(diphenylsulfonio) phenyl] sulfide bis-MF₆content is about 50-60 percent of the solids and the impuritiescontained therein have not been found to significantly affect thequalities of the photoinitiator. However, further purification toincrease the percentage of the photoinitiator in the product appears toincrease the activity thereof and may afford other advantages.

In the further purification, the dried product is dissolved in a solventtherefor selected from the group consisting of C₁ -C₃ alcohols, C₃ -C₆esters and C₂ -C₆ ketones. Exemplary of such solvents are methanol,methyl ethyl ketone, acetone, ethyl acetate and cellosolve acetate;methanol is preferred because of its low cost and handlingcharacteristics. To facilitate dissolution in the smallest volumepossible, the solvent is preferably heated to a temperature approachingits boiling point; for example, the preferred methanol is heated to atemperature of about 50°-65° C. Since the solvent must subsequently bestripped, it is obviously desirable to limit the volume thereof,preferably to the minimum required; with methanol, about 10-15 partswill dissolve 1 part of the crude dried product at the abovetemperatures. A negligible amount of a dark gum may not dissolve.

Following dissolution of the crude product in the solvent, the solutionis allowed to cool and the impurities settle out as a gum-likeprecipitate. The supernatant liquid is then separated from theprecipitate and is desirably evaporated to about 40-60 percent of itsoriginal volume. The concentrated liquid is then chilled to atemperature below about 10° C., at which point the substantially pureproduct precipitates as a white solid. The temperature to which theliquid is chilled will vary with the solvent selected; for methanol,temperatures between -5° and -30° are desirable to effect precipitationof substantially all of the product while avoiding precipitation ofimpurities.

The solvent is then separated from the product by filtration while stillchilled, and the product is then dried either in air or under vacuum attemperatures of up to about 110° C. for periods of 0.5-5.0 hours. Theresulting product is normally about 90-98 percent purebis-[4-(diphenylsulfonio) phenyl] sulfide bis-MF₆ with the remainingimpurities apparently being active photoinitiators and non-interfering.

The bis-[4-(diphenylsulfonio) phenyl] sulfide bis-MF₆ photoinitiatorsproduced by the present invention may be used to initiate polymerizationof any cationically polymerizable monomer formulation including epoxideformulations, vinyl ethers, cyclic ethers and ketones, lactones andoxetanes, styrenes, acrolein, vinylarenes such as 4-vinyl biphenyl,vinyl cycloaliphatics such as vinylcyclohexene, isobutylene, dienes suchas butadiene and isoprene, etc. The amount thereof incorporated in thepolymerizable formulation will normally fall within the range of 0.5-6.0percent by weight of the polymerizable monomers therein, and preferablyabout 1.0-4.0 percent.

The preferred polymerizable formulations are those in which epoxidemonomers are present and preferably those epoxide formulations whichcontain at least about 15 percent by weight of prepolymer having twoepoxycycloalkyl groups per molecule as will be described hereinafter.Alternatively, the epoxy formulation may be subjected to a controlledelevated temperature following exposure to radiation as will also bedescribed hereinafter.

The epoxidic prepolymer materials which are preferred for thepolymerizable formulations herein comprise any monomeric or oligomericmaterial containing at least one functional epoxy group or oxirane ringso that they may by polymerized upon opening of the oxirane ring. Inaddition, polymeric epoxy materials may be employed if they may bedispersed in the composition and are capable of undergoing furtherpolymerization to produce a solid polymer. The epoxy compounds may bealiphatic, cycloaliphatic, aromatic or heterocyclic.

The epoxidic prepolymer should contain no functional groups more basicthan the oxirane ring and should be a solvent for the initiator. Mostdesirably, the prepolymer should contain a reasonable percentage ofepoxy compounds containing two or more epoxy groups per molecule.

The polymerizable epoxy material will be epoxide resins used eithersingly or in combination and will have an average epoxide value of about0.1-1.0. The carbon chains having the epoxy groups may includeadditional substituents including ethers, esters, halogens, phosphates,and the like, and the compounds may include other polymerizablefunctional groups such as acrylates and silicones.

Typical epoxy materials are readily available commercially, the mostcommon being those which are the product of bis-phenol A withepichlorohydrin or those resulting from the reaction of epichlorohydrinwith a phenol/formaldehyde resin of relatively low molecular weight.Reference may be made to the HANDBOOK OF EPOXY RESINS by H. Lee and K.Neville (McGraw-Hill 1967) for various epoxides. In addition, thetechnical literature and patent literature both contain extensivediscussions of various epoxidic prepolymer materials which are useful inthe compositions of the present invention as will be demonstratedhereinafter.

In W. R. Watt U.S. Pat. No. 3,794,576, granted Feb. 26, 1974, there aredescribed radiation-sensitive epoxidic blends containing at least about15 percent by weight of an epoxidic ester having at least twoepoxycycloalkyl groups per molecule in order to achieve rapidpolymerization and curing of the composition rapidly upon exposure toultraviolet radiation or the like. Such compounds are convenientlyesters of an epoxidized cyclic alcohol and an epoxidizedcycloalkanecarboxylic acid or esters of an alkyl-substituted(epoxycycloalkane)methanol and a dibasic acid. A number of suitablecompounds are disclosed in the aforementioned Watt Patent. The advantageto the inclusion of the epoxycycloalkyl compounds is that polymerizationof the formulation will take place rapidly following exposure toradiation and without requiring temperature acceleration.

Although not essential and sometimes undesirable, the polymerizableepoxy composition may contain diluents to improve viscosity, and thesediluents may be reactive such as those produced by reaction of analcohol or a phenol with epichlorohydrin. Exemplary of reactive diluentsis the reaction product of nonylphenol with epicholorohydrin. The amountof diluent may vary from zero to as much as 45 percent of thecomposition if a reactive diluent is employed and is preferably lessthan 15 percent if nonreactive diluents such as di-butylphthalate areemployed.

For some applications, the composition may contain an inert pigment ordye to provide a desired coloration. Generally, such pigments and dyeswill comprise less than about 40 percent by weight of the composition.For certain applications, it may be desired to include an inert fillersuch as talc or silica, or polymers such as polyvinyl chloride, wheresuch fillers will not adversely affect the desired properties for thecured composition. They wll normally comprise less than 60 percent byweight and preferably less than 25 percent by weight of thepolymerizable composition.

The initiators of the present invention will decompose upon exposure toelectromagnetic radiation so as to provide a Lewis acid which iseffective to initiate polymerization of the cationically polymerizablecomposition. The term "Lewis acid" as used herein is intended toencompass compounds produced by decomposition and which will indirectlygenerate a Lewis acid to receive an electron pair from the monomer toinitiate polymerization, as for example from the oxygen of the oxiranering to open the oxirane ring. The classic Lewis acid precursordecomposition mechanism is described in the aforementioned Watt U.S.Pat. No. 3,794,576 with respect to a diazonium initiator.

The decomposition mechanism for triarylsulfonium salts to provide anindirectly formed Lewis acid has been postulated by Crivello et al in"Triarylsulfonium Salts: A New Class of Photoinitiators for CationicPolymerization" in JOURNAL OF RADIATION CURING, Vol. 5, page 2 (January1978). The authors postulate that the decomposition of the diaryl- andtriaryl- sulfonium salts produce a Bronsted acid which in turn providesa proton which will function as the Lewis acid to accept electrons froman oxygen of an oxirane ring in an epoxide monomer and initiatepolymerization in accordance with the following mechanism: ##STR1##

Regardless of the theory of the action embraced, it is apparent that thephotoinitiator is decomposing to generate in the reaction medium anelectron acceptor acting as a Lewis acid to open an oxirane ring in anepoxide compound or otherwise accepting an electron pair from othercationically polymerizable monomers and thereby initiating cationicpolymerization of the monomer material. The reaction then proceeds asadditional monomer units are activated until all of the monomer has beenpolymerized or until impurities interfere with the reaction mechanism.

As indicated, the photoinitiator is decomposed into a Lewis acid byexposure to electromagnetic radiation. Although electron beambombardment, X-ray radiation, and other similar forms of high energyradiation may be employed for this purpose, exposure to ultravioletradiation has been found highly satisfactory and is desirable forcommercial applications. The exposure to radiation normally required maybe of extremely short duration, periods of about one-half to threeseconds being normally adequate for most compositions depending upon theintensity of the radiation at the surface. However, for relatively thickdeposits of the composition, it may be desirable to extend the period ofexposure to four seconds or even more, to ensure adequate penetration ofthe radiation through the depth of the coating.

In the polymerization of epoxide prepolymer compositions which do notinclude the desirable epoxycycloalkyl compounds of the aforementionedWatt Patent to provide a rapid cure rate, it is possible to obtain rapidpolymerization of at least the surface portion by maintaining thecomposition within a relatively narrow elevated temperature range for aperiod of 0.5-5.0 seconds following initiation of exposure to theelectromagnetic radiation in order to achieve the desired rapidpolymerization of the epoxide prepolymer material to a tack-free surfacecondition within a period of less than 30 seconds. Although thiselevated temperature range may extend from 50° C. to as high as 90° C.,it is generally held within the range of 55°-75° C. to obtain thedesired rate of polymerization while avoiding adverse effects on theresulting polymer and the desired physical properties.

The temperature of the composition may be elevated to the desiredtemperature range by any suitable means including induction heating whena metallic substrate or container is employed; conductive heating;convection heating; and radiation heating by exposure to a source ofsuitable radiant heat such as infrared lamps. For convenience and forminimization of the equipment requirements, infrared radiation providedby suitable lamps is most desirably employed in conjunction with thesource of ultraviolet radiation used to produce decomposition of theinitiator.

The initiators of the present invention are particularly applicable tocoating compositions for making durable coatings for either aesthetic orprotective purposes. Epoxy compositions find particular advantage in thefield of graphic arts because of the resistance of the coating tosolvents and chemicals as well as to abrasion, because of the excellentadhesion to various surfaces including metals and because of the abilityto withstand drawing and forming operations.

Illustrative of the various aspects of the methods of the presentinvention are the following specific examples wherein all examples areparts by weight unless otherwise indicated.

EXAMPLE ONE

To a liter of water are added 40 grams of potassium hexafluorophosphatewith stirring to effect complete dissolution. To 5 liters of a water/iceslurry are added 130 grams of a 50 percent aqueous solution of acommercially purchased material comprising substantiallybis-[4-(diphenylsulfonio)phenyl] sulfide bis-chloride.

The solution of potassium hexafluorophosphate is added dropwise withvigorous stirring to the water/ice slurry, and a voluminous white,creamy precipitate is formed. Stirring is continued for one hour as theprecipitate increases in density. The mixture is then filtered on avacuum filter. The gelatinous precipitate is placed in a vacuum ovencontaining phosphorous pentoxide as a dessicant and dried to constantweight at a temperature of 50° C.

The yield is found to be 84.4 grams. Analysis indicates the product tobe about 50-60 percent bis-[4-(diphenylsulfonio) phenyl] sulfidebis-hexafluorophosphate. The product as formed by drying in the abovephase is a frangible white powder and may be readily dissolved in polarsolvents such as sulfolane and propylene carbonate.

An elemental analysis of the product indicates it to compare with thecalculated values for bis-[4-(diphenylsulfonio) phenyl] sulfidebishexafluorophosphate:

    ______________________________________                                        Element,% by wt. Found   Calculated                                           ______________________________________                                        Carbon           51.2    51.0                                                 Hydrogen         3.3     3.3                                                  Sulfur           11.8    11.4                                                 Phosphorus       6.6     7.3                                                  Fluorine         26.9    26.9                                                 ______________________________________                                    

Further purification of the product is effected in accordance with thefollowing procedure.

The dried product is dissolved in boiling methanol in a ratio of 10parts product to 100 parts by weight methanol. Following dissolution ofthe product, the solution is allowed to stand and cool to roomtemperature, at which point a brown to amber gum precipitates from thesolution. The gum precipitate is removed from the solution and thesupernatent liquid is evaporated to approximately 50 percent of itsoriginal volume. The evaporated solution is then chilled to atemperature of -15° C. at which point a white precipitate forms. Theprecipitate is filtered from the solution while the solution is stillchilled and the precipitate is air dried on the filter. It is furtherdried in a vacuum oven at a temperature of approximately 100° C. untilno further weight loss is observed.

Chromatographic analysis of the purified product indicates it to beapproximately 95 percent pure bis-[4-(diphenylsulfonio) phenyl] sulfidebishexafluorophosphate. The purified product has a melting point withinthe range of 231°-238° C. Elemental analysis is compared with thecalculated values for bis-[4-(diphenylsulfonio) phenyl] sulfidebis-hexafluorophosphate:

    ______________________________________                                        Element,% by wt. Found   Calculated                                           ______________________________________                                        Carbon           50.82   51.0                                                 Hydrogen         3.22    3.3                                                  Sulfur           11.36   11.4                                                 Phosphorus       7.04    7.3                                                  Fluorine         27.63   26.9                                                 ______________________________________                                    

EXAMPLE TWO

To evaluate the activity of the photoinitiator produced in accordancewith the present invention as compared with tri-phenylsulfoniumhexafluorophosphate, several different epoxide formulations areprepared, aliquots of each formulation utilizing as the photoinitiatortherein triphenyl sulfonium hexafluorophosphate (TPS), unpurifiedbis-[4-(diphenylsulfonio) phenyl] sulfide bis-hexafluorophosphate (BDS),or purified bis-[4-(diphenylsulfonio) phenyl] sulfidebis-hexafluorophosphate (BDS). In all instances, the photoinitiator isintroduced as a 33 percent by weight solution in propylene carbonate.

    ______________________________________                                        Component                 Amount                                              ______________________________________                                        FORMULATION I                                                                 3,4-epoxy cyclohexylmethyl-3,4-cyclohexane-                                   carboxylate                                                                   (sold by Ciba-Geigy under the designation CY-179)                                                       100.0   parts                                       Surfactant                0.5     parts                                       Photoinitiator Solution   6.0     parts                                       FORMULATION II                                                                Bis-(3,4-epoxycyclohexylmethyl)adipate                                        (sold by Union                                                                Carbide under the designation ERL 4299)                                                                 100.0   parts                                       Surfactant                0.5     parts                                       Photoinitiator Solution   6.0     parts                                       FORMULATION III                                                               3,4-epoxy cyclohexylmethyl-3,4-cyclohexane-                                   carboxylate                                                                   (sold by Ciba-Geigy under the designation CY-179)                                                       66.7    parts                                       Butanediol aliphatic diglycidyl ether (sold by Ciba-                          Geigy under the designation RD-2)                                                                       33.3    parts                                       Surfactant                0.5     parts                                       Photoinitiator Solution   6.0     parts                                       FORMULATION IV                                                                3,4-epoxy cyclohexylmethyl-3,4-cyclohexane-                                   carboxylate                                                                   (sold by Ciba-Geigy under the designation CY-179)                                                       80.0    parts                                       Epoxy (C.sub.12 --C.sub.14 alkylglycidyl ether                                (sold by Procter                                                              and Gamble under the designation Epoxide #8)                                                            20.0    parts                                       Surfactant                0.5     parts                                       Photoinitiator Solution   6.0     parts                                       ______________________________________                                    

The several epoxide formulations are applied to aluminum panels with a#4 wire wound rod and then placed upon a controlled temperature stagemaintained at 30° C. Approximately 30 seconds is allowed for the coatedpanel to reach temperature equilibrium with the stage, after which it isexposed to a source of electromagnetic radiation. A ball of absorbentcotton is placed against the exposed coating to determine the time forthe coating to become tack-free. The results of these tests are setforth in the following table:

    ______________________________________                                        COMPARISON OF CURE RATES                                                                    Tack-Free Time (Seconds)                                                            Following 2 se-                                                               conds exposure to                                                                          Following 6 se-                                                  medium pressure                                                                            conds exposure                                                   mercury arc  to 275-watt sun-                             Formulation                                                                            Photoiniator                                                                             at 30° C.                                                                           lamp at 30° C.                        ______________________________________                                          I      TPS        5            no cure                                               Crude BDS  1            1                                                     Purified BDS                                                                             1            1                                             II      TPS        6            no cure                                               Crude BDS  4            4                                                     Purified BDS                                                                             3            3                                            III      TPS        9            no cure                                               Crude BDS  3            4                                                     Purified BDS                                                                             2            3                                            IV       TPS        5            no cure                                               Crude BDS  3            4                                                     Purified BDS                                                                             2            2                                            ______________________________________                                    

From the test data set forth in the table above, it can be seen that thephotoinitiators produced by the method of the present invention offersignificant advantages in improved cure rates as compared with triphenylsulfonium hexafluorophosphate and also in greater sensitivity tosunlight. Other tests also indicate a response to light having wavelengths of greater than 300 nanometers which is significantly betterthan triphenyl sulfonium hexafluorophosphate and a better broad spectrumsensitivity than afforded by triphenyl sulfonium hexafluorophosphate.

Various tests conducted with respect to pot life indicate epoxy systemscontaining the photoinitiators produced by the method of the presentinvention exhibit significantly improved stability and accordingly,longer pot life.

As will be appreciated from an analysis of the method of the presentinvention, highly effective photoinitiators may be produced atrelatively low cost as compared with triphenyl sulfoniumhexafluorophosphate and other related onium photoinitiators. Thestarting materials are readily available and the procedures of thepresent invention are relatively simple and straightforward and involvesubstantially complete reaction of the components.

Thus, it can be seen from the foregoing detailed specification andexamples that the novel methods of the present invention enable thepreparation of a relatively economical and highly effectivephotoinitiator which exhibits improved spectral sensitivity as comparedto triphenyl sulfonium hexafluorophosphate and affords significantadvantages in cure rate. The methods utilize commercially availablematerials and are relatively simple. Since the product of the initialreaction following drying may be used without purification if sodesired, even greater economies may be effected. Although thepolymerizable compositions exhibit highly desirable shelf stability orpot life, polymerization proceeds rapidly following exposure toelectromagnetic radiation.

Having thus described the invention, I claim:
 1. In the method of makinga photoinitiator, the steps comprising:A. admixing a first reactantcomprising bis-[4-diphenylsulfonio) phenyl] sulfide bis-halide whereinhalide is chloride or bromide in aqueous solution with a second reactanthaving the formula T.MF₆ wherein M is one of phosphorus, antimony orarsenic, T is hydrogen or a metal whose halide salt is water soluble,said reactants reacting to produce a precipitate substantiallycomprising bis-[4-(diphenylsulfonio) phenyl] sulfide bis-MF₆, saidsecond reactant being present in an amount of at least about 2 moles permole of said solution providing about 0.5-2.0 moles water per mole ofsaid second reactant, said reactants being admixed in said aqueoussolution at a temperature of about 0°-50° C. for a period of timesufficient to permit said precipitate to form; B. recovering saidprecipitate by separation from said solution; and C. drying saidprecipitate at a temperature below about 50° C. for at least an initialperiod to dewater said precipitate and recover a frangible productcontaining at least 50 percent by weight bis-[4-(diphenylsulfonio)phenyl] sulfide bis-MF₆ based upon solids.
 2. The method in accordancewith claim 1 wherein said dewatered product is, subsequent to havingreached a point where it is readily frangible, dried further at atemperature of up to 110° C.
 3. The method in accordance with claim 1including the further steps of:A. dissolving the dried precipitate in asolvent therefor selected from the group consisting of C₁ -C₃ alcohols,C₃ -C₆ ketones, and C₂ -C₆ esters at an elevated temperature in excessof 50° C. below the boiling point of said solvent; B. cooling saidsolvent solution to form an insoluble precipitate; C. separating saidsolution from said precipitate; D. lowering the temperature of thesolution to below about 10° C. effective to precipitate outsubstantially pure product; E. recovering the precipitate; and F. dryingthe precipitate to produce a photoinitiator comprising substantiallybis-[(diphenylsulfonio) phenyl] sulfide bis-MF₆.
 4. The method inaccordance with claim 1 wherein said halide of said first reactant ischloride.
 5. The method in accordance with claim 1 wherein said phenylgroups of said first reactant and of said photoinitiator are halogensubstituted.
 6. The method in accordance with claim 1 wherein saidphenyl groups of said first reactant and of said photoinitiator areunsubstituted.
 7. The method in accordance with claim 1 wherein M isphosphorus.
 8. The method in accordance with claim 3 wherein saidsolvent is methanol and wherein the temperature of said solution islowered between 0° C. and -25° C.
 9. The method in accordance with claim8 wherein the elevated temperature at which said precipitate isdissolved in methanol is within the range of 50°-65° C.